1. Field of the Invention
The present invention relates to a transmission device for an ignition system of an internal combustion engine employed in automobiles or the like.
2. Description of the Prior Art
In general, ignition systems for internal combustion engines are made such that a high voltage output from an ignition coil is transferred through a transmission device to a spark plug.
FIG. 8 is a cross-sectional view showing a conventional direct plug-fitted transmission device for an ignition system of an internal combustion engine. In the figure, a cylinder head 1 is provided with a plug aperture 2. A spark plug 3 is screwed into a bottom portion of the plug hole 2. The spark plug 3 has an electrically conductive portion 3a.
An ignition system 4 of an internal combustion engine includes an ignition coil 5 and a transmission device 6. The transmission device 6 has an insulating coat 7 and a conductor 8. The insulating coat 7 is made of a silicone rubber which is an elastic insulating material. The insulating coat 7 has a cylindrical housing portion 7a that has a larger circumference at the spark plug 3 end and the end of which is formed into a thin-walled cylindrical portion 7b. A spring 11 is housed in the cylindrical housing portion 7a. The conductor 8 is turned out from a stiff round brass bar and covered with the insulating coat 7 thereby being insulated from the outside.
A plug-side elastic member 9 is provided with an inner cylindrical portion 9b having, at its center portion, a plug engaging aperture 9a in which the spark plug 3 is fitted. Further, the plug-side elastic member 9 is equipped with an outer cylindrical portion 9d located outside the inner cylindrical portion 9b with an engagement groove 9c being formed therebetween, and a reinforcing flange portion 9e formed at the top end portion of the outer cylindrical portion 9d as illustrated. These portions of the plug-side elastic member 9 are integrally formed using a silicone rubber with a given elasticity.
The thin-walled cylindrical portion 7b of the insulating coat 7 is inserted into the engagement groove 9c of the plug-side elastic member 9 so that a gap F exists between the inner circumference of the outer cylindrical portion 9d and the thin-walled cylindrical portion 7b, and a gap G exists between the outer circumference of the inner cylindrical portion 9b and the thin-walled cylindrical portion 7b. A silicone rubber-based adhesive agent 10 is packed in the gap F over the length L as illustrated. The adhesive length L is determined so that the plug-side elastic member 9 and the cylindrical housing portion 7a of the insulating coat 7 do not become disconnected from each other when the plug-side elastic member 9 is removed from the spark plug 3 by pulling on the insulating coat 7. Further, the adhesive agent 10 extends slightly above the flange portion 9e so that the cylindrical housing portion 7a and the outer cylindrical portion 9d thereby adhere to each other.
Furthermore, it is necessary to ensure that the conductor 8, the spring 11 and the electrically conductive portion 3a of the spark plug 3 are insulated from such grounded members as the cylinder head 1. To this end, the bonding adhesive agent 10 is packed so that a leakage gap does not remain between the outer circumference of the cylindrical housing portion 7a and the inner circumference of the outer cylindrical portion 9d. The cylindrical housing portion 7a is inserted into the engagement groove 9c after the adhesive agent 10 is applied onto its outer circumference over the length L. The spring 11 is formed by winding an elastic, conductive wire into a coil, and its upper end portion is then fixed to the conductor 8 to be electrically coupled thereto.
A coil-side elastic member 12 is made of an elastic polybutylene terephthalate (PBT). Further, although not illustrated, in the inside of the coil-side elastic member 12, one end portion of the conductor 8 is electrically connected with the high voltage side terminal of the ignition coil 5 and insulated from the outside. The transmission device 6 is composed of the insulating member 7, the conductor 8, the plug-side elastic member 9, the adhesive agent 10, the spring 11, and the coil-side elastic member 12.
In the transmission device 6 constructed as described above, the spark plug 3 is fitted into the plug engaging aperture 9a of the plug-side elastic member 9 with a given force, and the spring 11 is compressed to press the conductive portion 3a of the spark plug 3 to be electrically connected therewith.
The operation will now be described. A high voltage output from the ignition coil 5 travels from the conductor 8 through the spring 11 to the spark plug 3. The gap G present between the inner cylindrical portion 9b of the plug-side elastic member 9 and the thin-walled cylindrical portion 7b of the insulating coat 7 permits enlargement and deformation of the diameter of the inner cylindrical portion 9b when the spark plug 3 is fit into the plug-side elastic member 9, thus facilitating insertion and removal of the spark plug 3 into and from the plug-side elastic member 9.
A problem arises with such conventional transmission devices for ignition systems, however, in that a long adhesive range, i.e., adhesive length L, between the cylindrical housing portion 7a of the insulating coat 7 and the outer cylindrical portion 9d of the plug-side elastic member 9 is necessary, resulting in difficulties in achieving size-reduction, particularly the reduction of the axial dimension thereof, of the plug-side elastic member 9 made, for example, of an expensive silicone rubber. In addition, there is a possibility that the adhesive agent 10 could flow under the lower end portion of the thin-walled cylindrical portion 7b into the gap G between the inner cylindrical portion 9b of the plug-side elastic member 9 and the thin-walled cylindrical portion 7b of the insulating coat 7. If the adhesive agent 10 runs into the gap G and then hardens, difficulties are encountered in the enlargement and deformation of the inner diameter of the inner cylindrical portion 9b, so that more effort is required for the inserting it on or removing it from the spark plug 3, thus decreasing its workability. Accordingly, strict control of the application conditions and amount of the adhesive agent 10 is required.